The invention generally relates to the field of electrical switches. It is applicable, for example, to the design configuration of a drive train for the opening and closing process of a moving contact of a switching contact arrangement for an electrical switch, in which a coupling element, arranged between the moving contact and associated drive element and movable against a stop at the end of the opening process, includes an associated bouncing protection device including a catching element and a fixed-position opposing piece.
In a known electrical switch, the catching element includes a single-armed lever which is arranged such that it can pivot on the coupling element and is subject to the force of the setting spring. This means that the center of gravity of the catching element is not on its pivoting axis, so that, when the coupling element strikes the stop, the catching element can be pivoted to a catching position by the inertia force acting at its center of gravity (U.S. Pat. No. 4,468,533).
In the case of drive trains of this type, the elements of the drive rods are accelerated nonuniformly during the opening process and, in the process, the catching element (which has a working surface in order to form a temporary operative connection to the opposing piece) is pivoted from its rest position opposite the coupling element even before the coupling element strikes the stop, in such a way that it does not produce the necessary operative connection to the opposing piece. In the absence of the operative connection, the coupling element which strikes the stop can bounce back without any impediment to its position corresponding to the closed position of the switching contact arrangement, thus resulting in a risk of increased contact wear, or even of welding of the contacts. In this case, the bouncing of the coupling element when it strikes the stop is dependent on the level of the current to be interrupted and on the level of the electrodynamic forces which are caused by this current, act on the elements of the drive train and result in the elements of the drive train being moved more quickly during the opening process. For normal operating currents, the acceleration which is produced (the angular acceleration in the case of a coupling element which can move about a rotation axis) and hence the inertia force acting at the center of gravity of the catching element are less then, for example, in the event of high short-circuit currents.
In the case of the known switch (U.S. Pat. No. 4,468,533) which has been mentioned, the catching element (which is in the form of a lever) is firmly seated on a drive element in the form of a switching shaft. The catching element is in this case mounted on the coupling element such that the pivoting axis of the catching element is radially separated from the rotation axis of the switching shaft. In order to pivot the catching element to its catching position if the coupling element bounces, the radial extent of the catching element corresponds approximately to that of the coupling element, and thus requires a correspondingly large physical space and pivoting space within the drive train. In order to position the catching element accurately in its catching position, the catching element has a stop surface, which is associated with the opposing piece, in addition to the working surface.
The working surface, which runs at right angles to this stop surface, is pushed against the opposing piece if the coupling element bounces, assuming that the lever is positioned accurately in its catching position, thus preventing excessive bouncing of the coupling element. In order to position it in this way, it is necessary for the spring force of the resetting spring to be matched to the inertia force, which acts at the center of gravity of the catching element and is dependent on the acceleration of the coupling element, such that the stop surface of the catching element reliably makes contact with the opposing piece as a result of the coupling element striking the stop. The spring force can be matched in such a way only for a limited range of inertia forces. If the current forces which produce an inertia force are greater, with their value being outside this limited range, the angular acceleration and hence the angular velocity of the catching element when the stop surface strikes the opposing piece may be so great that the catching element bounces off the opposing piece (secondary bouncing), and the working surface cannot make the necessary operative connection to the opposing piece.
An embodiment of the invention is based on an object of designing the bouncing protection device so as to prevent the coupling element from bouncing back to a position which corresponds to the closed position of the switching contact arrangement, throughout the entire power range of the switch, that is to say from normal operating currents up to the maximum short-circuit currents.
According to an embodiment of the invention, an object may be achieved in that the catching element is additionally associated with a hold-back element, on which the catching element is guided during the opening process and, in the process, is held in its rest position opposite the coupling element until the coupling element reaches a predetermined position.
A catching element which is provided with a hold-back element in this way is guided during the opening process, irrespective of the nature and the value of the acceleration of the elements of the drive train, so as to reliably prevent uncontrolled deflection from its rest position as a result of non-uniform acceleration, such as that which occurs by way of example in the case of rapidly successive OFF-ON-OFF switching processes.
One development of an embodiment of the invention provides for the catching element to be in the form of a single-armed, hook-like lever, and for it to be possible for this lever to engage behind the opposing piece.
In an embodiment such as this, in which the working surface is drawn against the opposing piece when the coupling element bounces, the radial extent of the catching element may be designed to be so short that its installation space and pivoting space are not restricted by any other, for example by the drive element. In particular, the radical extent of the catching element in the case of a coupling element which is in the form of a lever arranged in a fixed position on a switching shaft may be shorter than the distance between the pivoting axis of the catching element and the rotation axis of the coupling element. The catching element may be fitted at different positions on the coupling element. A bouncing protection device such as this can thus advantageously be used in drive trains in which only a small physical space is available for the bouncing protection device. It can also be used in drive trains in which the coupling element carries out a linear movement.
The catching element may also be designed such that its working surface is pushed against the opposing piece in the event of bouncing, in a similar way to that with the known drive train (U.S. Pat. No. 4,468,533). However, for this purpose, the catching element must be in the form of a two-armed lever, in which the working surface is formed on one lever arm and the center of gravity is located in the area of the other lever arm. A two-armed lever such as this can likewise be used in drive trains in which the coupling element carries out a linear movement.
It is advantageous for the catching element to be able to pivot freely through a pivoting angle of at least 45xc2x0 in order to form the temporary operative connection, and for the working surface to extend over a circular arc angle of at least 20xc2x0. With a refinement such as this, catching takes place without the catching element running against a stop and, in consequence, not being able to bounce back itself. This is ensured in that, firstly, the catching element can pivot freely through a pivoting angle which is greater than that previously known and in that, secondly, the working surface extends over a relatively large circular arc angle. In consequence, in the event of high short-circuit currents, the catching element can pivot through a greater pivoting angle to its catching position than, for example, for normal operating currents. In any case, the catching element is held securely in its catching position by the interaction of the spring force of the resetting spring, which rises during pivoting of the catching element, and the friction force, which acts between the working surface and the opposing piece in the event of any bouncing of the coupling element which still occurs during the pivoting movement of the catching element.
A physically simple embodiment of the new drive train is characterized in that the hold-back element is in the form of a spring. In this case, the spring may be designed such that, once the coupling element has moved through a predetermined position, in particular when the moving coupling element strikes the stop, the catching element is accelerated by the spring in the direction of its catching position.
A further preferred embodiment of the new drive train is distinguished by the hold-back element in the form of a rigid part, which is provided with a slotted-link guide and is held on the drive train, in particular on a coupling member which is hinged on the coupling element. The rigid part may also be integrally formed, for example on the coupling member. In this case, the movement of the catching element can be controlled by a slotted-link guide curved in the form of a circular arc, if two coupling bolts pass through the rigid part, and with one coupling bolt coupling the coupling member to a contact lever support to which the moving contact is fitted, and the other coupling bolt coupling the coupling member to the coupling element.